This invention relates to the use of a combined membrane and sorption system for selectively removing certain ions from a liquid. In another aspect, the present invention is an apparatus for selective ion removal from liquids.
The selective removal of specific ions from a liquid or solution additionally containing other types of ions is necessary or desired in many areas of technology, such as when the selected ion is or can be converted to a valuable product or the selected ion is undesirable in the use for which the liquid is intended. For example, ions of valuable metals can be recovered from solutions via ion exchange and then further processed by conversion into the metal. Various processes are currently used to remove and/or concentrate ions for these reasons. Such processes include ion exchange, electrodialysis, reverse osmosis, chemical reduction and various biological techniques.
The level of some ions, such as nitrates, represent a health hazard when presented above a concentration considered safe for potable water. The high levels of nitrates found in ground water supplies in some locations necessitate treatment of these waters prior to consumption.
Adaptations and/or combinations of one or more of the known ion removal systems have been suggested for use in nitrate removal from water. In Wasserwirtschaft 75 (1985), No. 6, Pages 257 to 262, there is a summary of the standard nitrate removal systems. German Pat. Nos. 3,301,054 and 3,010,538 discuss strong-base anion exchange methods to remove nitrates from water.
The principal deficiency in these and other ion exchange techniques which attempt to remove nitrate ions is the failure of these systems to selectively and economically remove the nitrate ions when sulfate ions are present. Substantial portions of the capacity of the commercially available anion exchange resins are typically used to sorb sulfate ions. Regeneration of the resins is similarly inefficient because of the need to desorb sulfate ions.
In U.S. Pat. No. 4,479,877, there is a description of the removal of nitrates by ion exchange methods and the problems encountered in such processes. This patent describes a specialized strong-base anion exchange resin which is recommended for use in nitrate removal from drinking water because of its selectivity of nitrates in preference to sulfates. This type of resin is a very specialized resin and suffers from the various other disadvantages of ion exchange systems.